This invention relates generally to combine harvesters and, more particularly, to such machines fitted with some form of control system operable to adjust the amount of crop material passing through the machine by varying the ground speed of the combine.
The term "grain" will be used principally throughout this specification for convenience and it should be understood that the term is not intended to be limiting. Thus "grain" refers to that part of the crop which is threshed and separated from the discardable part of the crop material.
The purpose of control systems for combine harvesters is to maximize the capacity of the machine and yet minimize grain loss. Numerous control systems have been proposed but to date, no system has been commercially adapted for combine harvesters primarily because all systems so far proposed are impracticable in the sense that they do not meet the stated objective in a wholly satisfactory manner. The main factor affecting the practicability of combine harvester control systems is that of grain loss. On the one hand, a control system which does not take account of grain loss cannot seriously be entertained because grain loss is a highly critical factor in the harvesting operation. On the other hand, a control system which does take account of grain loss was suspect up till now in that there was no method of measuring grain loss in absolute terms. However, according to the co-pending European Patent Application No. 88.200.802.2, there now is provided a method for measuring grain loss in absolute terms by detector means as described in the co-pending European Patent Applications Nos. 88.200.801.4 and 88.200.803.0 and by a plurality of measurements of the grain separation at different locations along the length of the threshing and separating mechanism.
A problem with utilizing some form of grain loss measurement to effect machine control is however that the control loop is relatively slow because a given volume of crop material takes a finite time (typically 10 seconds in conventional combine harvesters and approximately 5 seconds in rotary combine harvesters) to progress through the machine before the grain loss associated with that volume is measured. With such a slow control loop, the control system can become unstable in that it oscillates or hunts between maximum and minimum control. This problem is also associated with any other parameter of the machine which may be used in the control system and which is related to some crop processing component located in the machine such that there is a significant time delay between crop material entering the machine and reaching that component.
Many of the control systems proposed to date adjust the ground speed of the combine harvester in order to control the intake of crop material (i.e. to control machine loading). The intake of a constant amount of crop material however, does not guarantee a constant grain loss level. Moreover, crop characteristics, weather conditions and machine adjustments have multiple influences on the machine loading detecting means which are not necessarily reflected in the grain separation process and hence in the grain loss. Control of the intake of the crop material therefore does not directly involve that the grain loss level is in control.
For the foregoing reason, other proposed control systems, such as disclosed in EP-B-No. 0.122.343, comprise a combined master and slave control loop in which the slave loop controls the intake of crop material (i.e. the machine loading) while the master loop controls the grain loss. The set value of the intake of crop material (i.e. the desired loading of the machine under the instant operating conditions) established by the master loop is combined with the output signal of the slave loop in order to control the ground speed of the combine. As such, an adaptive action is obtained in which the relation between the grain loss level and the intake level of crop material is continuously adjusted.
However, a problem again encountered when utilizing the intake of crop material and the grain loss in a combined adaptive control is that the parameters of the relation between intake of crop material and the grain loss vary too much in the same field because of the mentioned multiple influences. This fluctuating relation renders the control system unstable by oscillating between maximum and minimum set values for the intake of crop material, as set by the master loop.
Another problem with said combined adaptive control arises from the variation in the type of relation, depending on external influences. Under certain conditions, the relation is experienced as being exponential. More explicitely, this means that grain loss increases exponentially with a linear increase of the intake of crop material. It will be appreciated that as a result a dangerous situation is created in which an only small increase in machine throughput could lead to excessive grain loss. Also, in the mean time, a bad influence on the stability of the control system is experienced, which inevitably will start to oscillate between minimum and maximum set values.